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RF Cellular

VIEWS: 113 PAGES: 25

									                           RF
                        CELLULAR


2/28/2007                                         1




  The Basic Radio Concept of
  Cellular Communication
            Contents:

             1.   Frequency Concepts
             2.   Radio Propagation
             3.
             3    Maintain of Radio Propagation
                  Problem
             4.   Antenna
             5
             5.   Demographics Data
             6.   Model Tuning
             7.   Site Survey
             8
             8.   C            dC
                  Coverage and Capacityi
             9.   Cellular Concept


2/28/2007                                         2
                         1. FREQUENCY CONCEPTS

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 1. Frequency Concepts




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 1. Frequency Concepts


   FREQUENCY




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 1. Frequency Concepts


   WAVELENGHT




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 1. Frequency Concepts


   BANDWIDTH
    The          ff               ll     d
    Th amount of frequency range allocated to one
    application.



   CHANNELS
    Important factor in determining the capacity of a mobile
    system is the channel.
     y

    A channel is a frequency or set of frequencies which can be
                      transmission,                 receipt,
    allocated for the transmission and possibly the receipt of
    information.



2/28/2007                                                                 7




 1. Frequency Concepts


 A duplex channel, such as that used during a mobile call,
 uses two frequencies:
   One     h         d
 - O to the MS and one f         h MS.
                           from the MS
 - The direction from the MS to the network is referred to as uplink.
 - The direction from the network to the MS is referred to as downlink.




2/28/2007                                                                 8
 1. Frequency Concepts



   DUPLEX DISTANCE
   Full duplex requires that the uplink and downlink transmissions
   must be separated in frequency by a minimum distance.
   This is the duplex distance. Without it, uplink and downlink
   frequencies would interfere with each other.




2/28/2007                                                               9




 1. Frequency Concepts



   CARRIER SEPARATION
    Every mobile system includes a carrier separation.
    This is the distance on the frequency band between channels
    being transmitted in the same direction.

    This is required in order to avoid the overlapping of information
                                      channel.
    in one channel into an adjacent channel




2/28/2007                                                               10
 1. Frequency Concepts



   ACCESS METHOD
   Most di i l cellular systems use of Ti
   M     digital ll l                         Division Multiple Access
                                      f Time Di i i M l i l A
   (TDMA) to transmit and receive speech signals.
   TDMA, one carrier is used to carry a number of calls, each call
   using that carrier at designated periods in time.
   These periods of time are referred to
    as time slots.

   The information sent during one time
                    burst.
   slot is called a burst
   In GSM, a TDMA frame consists
   of 8 time slots.
   This           h          di
   Thi means that a GSM radio carrieri
   can carry 8 calls.


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                              2. RADIO PROPAGATION




2/28/2007                                                                12
 2. Radio Propagation


   PATH LOSS
    Path loss occurs when the received signal becomes
    weaker and weaker due to increasing distance between
    MS and BTS, even if there are no obstacles between
    the transmitting (T ) and receiving (R ) antenna.
    th t       itti (Tx) d        i i (Rx) t




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    2. Radio Propagation


     SHADOWING
     Shadowing occurs when there are physical obstacles including
     hills and buildings between the BTS and the MS. The obstacles
     create a shadowing effect which can decrease the received
      i
     signal l
     strength. When the MS moves, the signal strength fluctuates
     depending on the obstacles between the MS and BTS.


                                                  A signal influenced
                                                  by fading varies in
                                                  signal strength.

                                                  Drops in strength
                                                  are called fading
                                                  dips.

2/28/2007                                                               14
    2. Radio Propagation


     MULTIPATH FADING

      Multipath fading occurs when there is more than one
      transmission path to the MS or BTS, and therefore more than
      one signal arriving at the receiver.
      This may be due to buildings or mountains, either close to or
      far from the receiving device.


      Rayleigh fading and time dispersion are forms of
      multipath fading.
           p         g




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    2. Radio Propagation


     Rayleigh (Fast) Fading
     A signal takes more than one path between the MS and BTS
     antennas. In this case, the signal is not received on
     a line of sight path directly from the Tx antenna. Rather, it is
                   buildings.
     reflected off buildings

     Rayleigh fading occurs when the obstacles are
      l    to the    i i     t
     close t th receiving antenna.




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    2. Radio Propagation


     Log‐Normal (Slow) Fading
                                               terrain.
            Caused by larger obstructions like terrain




2/28/2007                                                                   17




    2. Radio Propagation


     Time Dispersion
      Time dispersion is another problem relating to multiple paths
      to the Rx antenna of either an MS or BTS.
      However, in contrast to Rayleigh fading, the reflected signal
                                                  antenna.
      comes from an object far away from the Rx antenna


                                                 Time dispersion causes
                                                 Inter-Symbol Interference
                                                 (ISI) where consecutive
                                                 symbols (bits) interfere with
                                                 each other making it
                                                 difficult for the receiver to
                                                 d        i      hi h
                                                 determine which symbol ib l is
                                                 the correct one


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    2. Radio Propagation

     Time Alignment
     Each MS on a call is allocated a time slot on a TDMA frame.
     This is              f i    during hi h h
     Thi i an amount of time d i which the MS transmits   i
     information to the BTS.
     The information must also
     arrive at the BTS within
     that time slot.
     The time alignment p
                  g       problem
     Occurs when part of the
     information transmitted
     by an MS does not arrive
     within the allocated time slot.
     Instead, that part may arrive
                           slot,
     during the next time slot and may
     interfere with information
     from another MS using that
       th ti      l t
     other time slot.
2/28/2007                                                                19




    2. Radio Propagation

     Combined Signal Loss
      Some of these problems may occur at the same time  time.
      An illustration of what the signal strength may look like at the
      MS Rx antenna when moving away from the BTS Tx antenna
                    Figure.                       loss,
      is shown in Figure The problems of path loss shadowing and
      Rayleigh fading are present for this transmission path.




2/28/2007                                                                20
    2. Radio Propagation

     PROPAGATION MECHANISM




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    2. Radio Propagation




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    2. Radio Propagation




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                           3. MAINTAIN of RADIO
                              PROPAGATION PROBLEM

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    3. Maintain of Radio Propagation Problem

     Channel Coding
                 transmission,
      In digital transmission the quality of the transmitted signal is
      often expressed in terms of how many of the received bits are
      incorrect. This is called Bit Error Rate (BER). BER defines the
      percentage of the total number of received bits which are
      incorrectly detected.




       Channel coding is used to detect and correct errors in a
       received bit stream.
       It dd bits
       I adds bi to a message. Th          bits     bl   h
                                    These bi enable a channel l
       decoder to determine whether the message has faulty
       bits, and to potentially correct the faulty bits.

2/28/2007                                                                25




   3. Maintain of Radio Propagation Problem

     Interleaving
      Used to separate consecutive bits of a message so that these
      are transmitted in a non-consecutive way.




2/28/2007                                                                26
    3. Maintain of Radio Propagation Problem

     Antenna Diversity
                      y                         g          g
     Antenna diversity increases the received signal strength byy
     taking advantage of the natural properties of radio waves.
     There are two primary diversity methods: space diversity and
     polarization diversity.

            Space Diversity
            Mounting t
            M                 i      t      i t d f            th t
                 ti two receiver antennae instead of one. If the two RxR
            antennae are physically separated, the probability that both of
            them are affected by a deep fading dip at the same time is
            low.

            Polarization Diversity
             With polarisation diversity the two space diversity antennae
             are replaced by one dual polarized antenna. This normal size
                                                            arrays.
             but contains two differently polarized antenna arrays
2/28/2007                                                                     27




    3. Maintain of Radio Propagation Problem




2/28/2007                                                                     28
    3. Maintain of Radio Propagation Problem

     Adaptive Equalization
         p      q                          p         y     g
      Adaptive equalization is a solution specifically designed to
      counteract the problem of time dispersion.




2/28/2007                                                                         29




    3. Maintain of Radio Propagation Problem

     Frequency Hopping
      Rayleigh fading is frequency dependent. This means that the fading
      dips occur at different places for different frequencies. To benefit from
      this fact, it is possible for the BTS and MS to hop from frequency to
      frequency during a call. The frequency hopping of the BTS and
        S
      MS is synchronized.




2/28/2007                                                                         30
    3. Maintain of Radio Propagation Problem




2/28/2007                                                            31




    3. Maintain of Radio Propagation Problem

     Timing Advance
      Timing advance is a solution specifically designed to
                   h      bl    f i    li         It   k by
      counteract the problem of time alignment. I works b
      instructing the misaligned MS to transmit its burst
      earlier than it normally would.

      In GSM, the timing advance information relates to bit times.
            ,            y
      Thus, an MS may be instructed to advance its transmission
      by a certain number of bit times. The maximum in GSM is 63
      bit times. This is one of the parameters that limits the GSM
                                        radius.
      cell size to a maximum of 35 km radius




2/28/2007                                                            32
    3. Maintain of Radio Propagation Problem




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             4. ANTENNA

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    4. Antenna

     Introduction
     The requirements on the antennas needed for the ever
     expanding networks are becoming continually higher:

               t i tl defined di ti         tt     for      t    t
            • strictly d fi d radiation patterns f a most accurate
              network planning.
            • growing concern for the level of inter-modulation due to
              the radiation of many HF-carriers via one antenna.
            • dual polarization
            • electrical down-tilting of the vertical diagram.
                                    g                    g
            • unobtrusive design.




2/28/2007                                                                35




    4. Antenna

     Definition
      Polarization
      Polarization       be defined
      P l i i can b d fi d as the di          i    f  ill i     f h
                                    h direction of oscillation of the
      electrical field vector.
      Mobile communications: vertical polarization

      Propagation Pattern
                         p p g
      In most cases the propagation characteristic of an antenna can
      be described via elevations through the horizontal and vertical
      radiation diagrams. In mobile communications this is defined by
                                      (H plane)
      the magnetic field components (H-plane) and the electrical field
      components (E-plane). Very often a 3-dimensional description is
      chosen to describe a complex antenna.




2/28/2007                                                                36
    4. Antenna


    Half-Power-Beam-Width

    This term defines the aperture
    of the antenna. The HPBW is
    defined by the points in the
                            diagram
    horizontal and vertical diagram,
    which show where the radiated
    power has reached half the amplitude
                          direction.
    of the main radiation direction
    These points are also called 3 dB
    points.




2/28/2007                                                               37




    4. Antenna



   Gain
                                                     direction.
   An antenna without gain radiates energy in every direction An antenna
   with gain concentrates the energy in a defined angle segment
   of 3-dimensional space. The l/2-dipole is used as a reference for
   defining i
   d fi i gain.

   Calculation:
   Gain (with reference to the isotropic radiator dBi) = Gain (with
   reference to l/2-Dipole dBd) + 2.15 dB
   The gain of an antenna is linked to the radiation characteristic of the
        g
   antenna. The gain can be roughly calculated by checking the HPBW`s
   in the horizontal and vertical planes.




2/28/2007                                                               38
    4. Antenna


     Impedance
            q     y p            p              p
     The frequency dependant impedance of a dipole or antenna is
     often adjusted via a symmetry or transformation circuit to meet
     the 50 Ohm criterion. Adjustment across a wider frequency
     range is achieved using compensation circuits.


     VSWR /Return Loss
     An impedance of exactly 50 Ohm can only be practically achieved
     at one frequency. The VSWR defines how far the impedance
     differs from 50 Ohm with a wide-band antenna. The power
                                                          p
     delivered from the transmitter can no longer be radiated without
     loss because of this incorrect compensation. Part of this power is
                                                     transmitter.
     reflected at the antenna and is returned to the transmitter




2/28/2007                                                                 39




    4. Antenna




2/28/2007                                                                 40
    4. Antenna

     BTS Antennas
     • Omnidirectional Antennas




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    4. Antenna




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    4. Antenna




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    4. Antenna




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    4. Antenna




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    4. Antenna




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    4. Antenna




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    4. Antenna




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    4. Antenna




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